If you wish to troubleshoot, I suggest the following steps:
1) Remove the CPU. With a magnifying glass, examine the pins of the CPU socket, looking for bent ones. If damage is found, you can try to straighten the pin(s).
2) Remove the motherboard from the computer.
3) Replace the following:
a) A single memory module
b) The video card
c) The CPU/Heatsink/Fan (applying suitable heatsink compound)
4) Keeping the motherboard OUTSIDE THE CASE, temporarily attach the power supply connectors, a keyboard, and the "Power On" switch leads to the motherboard.
5) See what happens. It's possible that something got bumped during the CPU changeouts.
6) If this doesn't work, consider trying a different power supply.